1. Field of the Invention
The invention relates to hybrid vehicle powertrains with an internal combustion engine, a multiple-ratio transmission and an electric starter/alternator motor between the engine and the transmission.
2. Background Art
A contemporary automotive vehicle powertrain typically includes a hydrokinetic torque converter disposed between a transmission with multiple-ratio gearing and an internal combustion engine. The turbine of the torque converter transfers power to the power input element of the multiple-ratio gearing.
The presence of the torque converter in a vehicle powertrain of this type introduces hydrokinetic power losses, particularly during vehicle start up and advanced throttle downshifts. The power losses are manifested by thermal energy build up in the hydrokinetic torque converter fluid, which requires a heat exchanger to maintain an acceptable fluid temperature. Attempts have been made to reduce power losses normally associated with torque converter automatic transmission by eliminating the torque converter and replacing it with an electric motor (starter/alternator). A powertrain configuration of this type takes advantage of the performance of an internal combustion engine with the advantages of an electric motor that complements the speed and torque characteristics of the engine. It improves fuel economy of the powertrain while reducing undesirable exhaust gas emissions. It also permits the engine to be deactivated when the vehicle is at rest. The motor, which could be a high voltage induction motor, is available to provide added performance. Further, the engine can be disconnected from the power flow path of the powertrain as the induction motor supplies driving torque. When the motor is not required for providing driving torque, it can function as an alternator.
Further, in a powertrain of this type, the kinetic energy stored in a moving vehicle can be collected by charging a high voltage battery during deceleration.
In a conventional powertrain with a hydrokinetic torque converter, shift quality may be improved by controlling transmission input torque. Transmission input torque is reduced during a power on upshift by retarding the engine spark to reduce engine output torque. This improves both the durability of the oncoming friction element and the smoothness of the upshift event. Torque modulation using spark retard will satisfy the timing and repeatability requirements to satisfy shift quality targets, but this wastes a small amount of energy during the shift that heats the exhaust gases. Further, it can only reduce torque, not increase it. Torque modulation also can be accomplished by using a fuel cut off to reduce engine torque, but restoring engine torque following a shift event often is not repeatable using fuel control.
In a conventional powertrain using a hydrokinetic torque converter, a coast mode occurs whenever the accelerator pedal is off, both with and without braking. As the vehicle slows, a coasting downshift must be done to keep the engine speed within the desired range. In the case of a coasting downshift for a hybrid electric vehicle, the regenerative braking function coincides with the coast mode. Since the motor is located between the engine and the transmission, the coast downshift is done with a significant level of negative torque at the input to the transmission. This is an operating condition that differs from a condition found in a conventional powertrain, where coasting downshifts are done with only a slight negative or positive torque at the transmission input.